This invention is concerned with apparatus for heating electrically conductive flowable media, typically liquids. It is commonly necessary to heat flowable media to elevated temperatures, for example to perform a chemical reaction or, where the flowable medium is a foodstuff, to cook or sterilise the foodstuff. Conventional methods of applying heat to flowable media such as liquids include plate heat exchangers, autoclaves, steam or hot water-jacketed vessels and hot-air ovens. Such conventional methods may provide certain problems arising, for example, from the uneven temperature distribution established in the media according to the laws of heat conduction from a hot surface into the cold medium. Further, the rate of heat input into the medium is dependent on the surface area of the heating element in contact with the medium and the maximum temperature to which the medium immediately adjacent the heating surface can be raised without some deleterious effect. For example, in the heating of liquid foodstuffs such as dairy products requiring pasteurisation or sterilisation, the product is liable to protein denaturation at an excessively hot heat exchange surface. Furthermore, the surface area of the heat exchanger is also restricted since high surface areas entail fine mesh heat exchange structures which restrict flow of the media and can readily be fouled by viscous media or media with solid particles entrained. Furthermore, fouling of the heat exchange surface can occur due to the excessive temperature at the heat exchange surface, which again leads to a diminished heat transfer rate from the fouled surface which progressively compounds the problem. Fouling also increases the pressure drop required across the heat exchanger to ensure flow of the medium.
Canned produce containing meat, vegetable or fruit components in a liquid base are commonly autoclaved to achieve sterility. Over-cooking can occur at the can's surface with consequent loss of texture, flavour and nutritional value. Further, certain delicate foods containing, for instance, a yogurt or starch base, could benefit from a sterilising process offering very rapid heating rates which themselves can be difficult to achieve with known hot surface heat exchanger techniques.
It is already known to heat flowable media such as liquids by causing an electric current to flow directly in the media between pairs of electrodes. Such direct electrical ohmic heating of the medium can permit high rates of heat input to the medium, enabling relatively rapid heating rates. The basic problem of heating by conduction from a hot heat exchanger surface is also obviated. Proposals for ohmically heating liquids, specifically for pasteurising milk, are described in the article entitled "Pasteurization of Milk by Electricity", by F. H. McDowall, pages 275 to 291 of The New Zealand Journal of Science and Technology, February 1929. The "Electro-Pasteur" described in this article ohmically heats milk by electric currents flowing between electrodes spaced at intervals along a pipe through which the milk is flowing. Thus, the electric current flows parallel to the direction of flow of the milk.